It’s not known if the active-duty troops will be deployed in the San Diego sector, in part because it’s not clear where the caravan will go — or how many people will be in it by the time it crosses all the way through Mexico.

In the spring, an earlier caravan eventually brought about 200 people en masse to the San Ysidro port of entry to request asylum.

That caravan prompted the Trump administration to ask for National Guard help at the border. California Gov. Jerry Brown authorized up to 400 troops to work in support roles targeting transnational crime. Under the agreement, which runs through March, troops are stationed in San Diego, El Centro and Riverside.

Another 1,700 guard members are deployed in other states.

Under Brown’s authorization, the troops in California are not supposed to be used for immigration enforcement, but court documents in illegal-entry cases show that in some instances soldiers monitoring patrol cameras have helped with apprehensions.

Other presidents have also dispatched the National Guard to the border. Under President George W. Bush, guard members were stationed there for about two years beginning in 2006, and under President Barack Obama they assisted for about a year starting in 2010.

Those missions cost a total of about $1.3 billion, according to a federal report.

The use of active-duty troops along the border is controversial in part because of a fatal 1997 encounter between a Marine patrol from Camp Pendleton and an 18-year-old high school student in Texas.

Esequiel Hernandez Jr. was herding his family’s goats along the Rio Grande near Redford, a town about 200 miles southeast of El Paso, when he crossed paths with four camouflage-clad Marines who were watching for drug smugglers.

Hernandez fired twice at the patrol with a .22-caliber rifle, and one of the Marines fatally shot him, according to investigators. A military report issued a year later concluded that the Marines had not been adequately trained for an armed operation among civilians.

The U.S. government paid the teen’s family $1.9 million.

According to published accounts, the troops being readied for the new deployment will be used in support roles so they won’t violate the Posse Comitatus law, which dates from the late 1870s and prohibits the military from doing domestic police work.

But critics of the plan believe any use of the military sends the wrong message.

“The southern border should be a place of encounter, opportunity and hope — not a place of hate, exclusion and out-of-control militarization,” said Vicki B. Gaubeca, director of the Southern Borders Communities Coalition. “To turn these desperate parents, who are only trying to provide safety for their children, into political pawns is to betray our sacred promise to treat people humanely in their time of need — regardless of where they come from.”

Peter Nunez, a former U.S attorney in San Diego who supports stricter immigration enforcement at the border, said he thinks the military should play a role.

“I have supported using the military on the border since the 1980s, to render whatever support is necessary to the Border Patrol,” he said.